Hot melt adhesive dispensing systems generally include a dispenser coupled with one or more dispensing guns, heated hoses fluidly connected to the dispensing guns, and a dispensing unit for melting and supplying heated liquid adhesive to the guns through the heated hoses. The dispensing units of conventional hot melt adhesive systems feature a heated tank for melting and heating adhesive material received into the tank in solid or semi-solid form, a pump, a pump manifold for pumping the molten hot melt adhesive from the tank to outlet ports coupled with the heated hoses, and a controller. Among other system operations, the controller regulates the power supplied to the tank heater and heated hoses to maintain the liquid adhesive at an appropriate viscosity and temperature, depending on the application.
Traditional hot melt adhesives are thermoplastic adhesives that are widely used in industry for adhesively bonding many diverse types of products. Such traditional thermoplastic adhesives are solid at room temperature and must be heated to cause a phase transition to a liquid or semi-solid state for promoting flowability and dispensability. In contrast to traditional thermoplastic adhesives, liquid hot melt adhesives have been recently developed that exist in a flowable form at room or ambient temperature without heating to precipitate a phase transition.
Liquid hot melt adhesives are characterized by properties that differ significantly from the properties of traditional hot melt materials. Traditional hot melt materials are converted from a room temperature solid to a flowable form in a heated melter and subsequently pumped through a heated hose to a heated manifold and applicator or gun. In contrast, liquid hot melt adhesives are flowable and have a relatively low viscosity at ambient or room temperatures and pressures, form a highly viscous material with adhesive properties similar to traditional hot melt materials when activated by, for example, exposure to elevated temperatures and/or pressures, and solidify upon cooling after being applied to a substrate. The activated and solidified material behaves like a traditional thermoplastic hot melt adhesive and possesses similar bonding characteristics. An exemplary liquid hot melt adhesive, which is disclosed in U.S. Patent Application Publication No. 2004/0029980, consists of discrete particle components dispersed in a carrier fluid and is heat activated.
Liquid hot melt adhesives may be activated by, for example, heating shortly before being dispensed from the dispensing gun(s) onto a substrate. The activation may occur at or near the dispensing gun. Alternatively, the activation may occur at any location between the tank and dispensing gun sufficiently close to the dispensing gun so that the viscosity of the activated liquid hot melt adhesive remains low enough to permit flow from the activation site to the dispensing gun. Although liquid hot melt adhesive is not heated at the tank as are traditional thermoplastic adhesives, adhesive dispensing systems nonetheless require a pump, such as a piston pump, for pumping the liquid hot melt adhesive in a nonactivated state from the tank to the dispensing guns.
One difficulty observed when pumping liquid hot melt adhesives and, for that matter, when pumping other traditional thermoplastic adhesives, is that these materials tend to coagulate and form a solid tacky coating on moving components located in the liquid path inside the pump. The residual coating may adversely affect the operation of these moving components and, hence, pump operation. The balls of pump check valves are one type of moving component particularly affected by the formation of these coatings. A coated check valve ball may tend to adhere and stick to its seat, which makes pumping inefficient. Eventually, the presence of the coating may result in a total pump failure that may be remedied only by completely disassembling and cleaning the pump to remove the coating.
A need therefore exists for a dispensing unit of a hot melt adhesive dispensing system having a pump equipped with moving components resistant to the adverse effects of coating by coagulated liquid hot melt adhesive as described above.